There are known several types of uninterruptible power supplied (UPS), the most common of which are the "on-line" and "off-line" UPS. The "on-line" UPS is essentially composed of a battery charger powered by the AC line, a storage battery and an inverter for converting the DC voltage from the battery to an AC voltage to a load. The disadvantages of this kind of arrangements are the high cost of the controlled battery charger, which, in addition, is also a cumbersome device, and the relatively low efficiency of the arrangement.
The "off-line" UPS is a slightly more sophisticated arrangement wherein, in addition to the "on-line" components, it also comprises an AC voltage monitor and control circuit. The "off-line" UPS is thereby synchronized with the frequency and phase of the line and during normal operation, the load is powered directly by the line. When the AC line voltage drops below a predetermined level, or is altogether cut off, the "off-line" UPS automatically comes on line. The switch over time is of at least 5 msec. This minimal transit time of 5 msec. is a too long period for sensitive systems such as computers, and moreover, this "off-line" arrangement does not provide a constant output voltage but rather it merely follows the line fluctuations and, also, the switch-over action itself causes voltage fluctuations at the load.